c/file/download: display an error if highlighting fails
c/file/download: display an error if highlighting fails

Previously we displayed an empty page since stderr usually goes to the
error log of the web server.

Signed-off-by: Florian Pritz <753f544d2d01592750fb4bd29251b78abcfc8ecd@xinu.at>

Bluewind/filebin,Bluewind/filebin,jreinert/filebin,jreinert/filebin,Bluewind/filebin,Bluewind/filebin,jreinert/filebin,jreinert/filebin,Bluewind/filebin,jreinert/filebin,jreinert/filebin